PLCD,anovelmagneticdisplacementsensor.pdf
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Sensors and Actuators A, 25-27 (1991) 217-282
PLCD, A Novel Magnetic Displacement Sensor
0. ERB, G. HINZ and N. PREUSSE
Vacuumschmelze GmbH, Griiner Weg 37, D-6450 Hanau (F.R.G.)
217
Abstract
PLCD is the abbreviation of a new type of
displacement sensor operating on a magnetic-
inductive principle. It uses a flat bar as mag-
netic core wound according to the principle
of differential transformers and a permanent
(‘driving’ or ‘marking’) magnet guided along
the wound core to determine a position or
displacement. PLCDs are characterized by an
application-oriented accuracy level combined
with a versatile individually adaptable design
and mode of operation as well as a simple,
robust construction. They give access to fields
of application which normally cannot be cov-
ered by other displacement sensors for techni-
cal or economic reasons. Their main feature
is the mechanical separation of the marking
magnet attached to the position-marking ele-
ment from the sensor (sensor component).
Besides this, the fact that in certain circum-
stances their properties are largely indepen-
dent of the precise distance between magnet
and sensor can be utilized with advantage.
1. Introduction
The measurement of displacement or posi-
tion is one of the most important and oldest
tasks in sensor technology [l]. As a conse-
,quence, displacement and position sensors
have been developed along widely varying
principles and in diverse designs. However, a
brief survey of existing solutions reveals that
none of them adequately fulfils all the crucial
application criteria such as: reliability, long
lifetime; accuracy, resolution, frequency be-
haviour etc.; flexibility, application-specific
adaptability; and cost effectiveness.
0924424719 I /$X50
This problem can be illustrated by the
well-known optical methods which feature
accuracy and high resolution but are fre-
quently unreliable, for instance their applica-
tion under certain environmental conditions
is extremely limited. On the other hand, sen-
sors working on an inductive-magnetic prin-
ciple have proved to be extremely reliable but
are often difficult to adapt to practical use.
Against this background, development
work was fairly recently initiated on a new
displacement sensor operating on a contact-
less principle, i.e., with almost unlimited life-
time, and so flexible in design that it can be
adapted to totally different application situa-
tions. A further target for this development
was to achieve an attractively priced solution.
The following deals with the operating
principle and the most important properties
of the solution, gives examples of possible
applications.
2. Basic Construction
2.1. Operating Principle
As a result of demands for extreme life-
time and reliability, a magnetic-inductive
method was the obvious first choice. In this
category, we have the well-known actuators
based on a transformer or inductive principle
operated by a moving magnetic core (e.g.,
LVDT [2,3] or FLDT [4]), or those using a
shorting ring as a position marking element
[5]. To increase the versatility and to simplify
the design, experiments were run on the
possibility of combining the favourable differ-
ential-transformer configuration with a per-
manent magnet acting as position marker [6].
A straightforward configuration as shown
0 Elsevier Sequoia/Printed in The Netherlands
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